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1.1.1

Cardiac action potential – contractile cells

The cardiac action potential differs significantly depending on the function of the cardiac myocyte (i.e. excitatory/pacemaker or contractile). The action potential of contractile cardiac myocytes has 5 phases characterized by a stable resting membrane potential and a prolonged plateau phase.

 Phase 0 – rapid depolarization as membrane permeability to potassium decreases and fast sodium channels open.

 Phase 1 – early rapid repolarization as sodium permeability decreases.

 Phase 2 – plateau phase. A continued influx of calcium through L-type (long opening, voltage-gated) calcium channels maintains depolarization for approximately 300 ms.

 Phase 3 – rapid repolarization due to inactivation of calcium channels and ongoing efflux of potassium.

 Phase 4 – restoration of ionic concentrations, thereby restoring the resting membrane potential of approximately –90 mV.

For the majority of the action potential, contractile myocytes demonstrate an absolute refractory period (beginning of phase 0 until close to end of phase 2). During this time no stimulus, regardless of the magnitude, can incite further depolarization. A relative refractory period exists during phase 3. A supramaximal stimulus during this period will result in an action potential with a slower rate of depolarization and smaller amplitude, producing a weaker contraction.

Anti-arrhythmic drugs and the myocardial action potential

Anti-arrhythmic drugs (see Section 1.1.22 – Vaughan–Williams classification) that alter ion movement are used to alter action potentials to prevent or terminate arrhythmias.

 In contractile cells, sodium channel blockers (Vaughan–Williams Class 1) reduce the slope of phase 0 and the magnitude of depolarization. They also prolong the refractory periods by delaying the reactivation of sodium channels.

 Potassium channel blockers (Vaughan–Williams Class 3) delay phase 3 repolarization. This lengthens the duration of the action potential and the refractory periods.

1.1.2

Cardiac action potential – pacemaker cells

The pacemaker potential is seen in cells of the cardiac excitatory system, namely the sinoatrial (SA) and atrioventricular (AV) nodes. Action potentials of cardiac pacemaker myocytes have 3 phases (named out of numerical order to coincide with contractile myocyte action potentials) and are characterized by automaticity, due to an unstable phase 4, and a lack of plateau phase.

 Phase 4 – spontaneous depolarization. Sodium moves into myocytes via ‘funny’ voltage-gated channels that open when the cell membrane potential becomes more negative, immediately after the end of the previous action potential. Calcium also enters the cell via T-type channels (T for transient).

 Phase 0 – rapid depolarization occurs once the threshold potential (approximately −40 mV) is reached. L-type calcium channels open and calcium enters the cell.

 Phase 3 – repolarization occurs as potassium permeability increases, resulting in potassium efflux.

Compared to contractile myocytes, pacemaker myocyte action potentials:

 are slow response

 have a less negative phase 4 membrane potential

 have a less negative threshold potential

 have a less steep slope of rapid depolarization (phase 0).

Regulation by the autonomic nervous system

The cardiac excitatory system demonstrates inherent pacemaker activity. The rate of depolarization and duration of action potential are influenced by the autonomic nervous system. In the denervated heart, the SA node depolarizes at a rate of 100 bpm. At rest, parasympathetic activity dominates and reduces SA nodal depolarization. Parasympathetic activation leads to an increase in potassium efflux while reducing sodium and calcium influx. These alterations in ionic conductance result in a more negative phase 4 membrane potential, a decrease in the slope of phase 4 and, overall, an increase in the time to reach the threshold potential. Conversely, sympathetic activation increases the rate of pacemaker depolarization by reducing potassium efflux and increasing sodium and calcium influx.

1.1.3

Cardiac action potential – variation in pacemaker potential

The pacemaker potential is seen in cells in the SA and AV nodes. It is a slow positive increase from the resting potential that occurs at the end of one action potential and before the start of the next. The pacemaker action potential differs from those seen in other cardiac cells because it lacks phases 1 and 2 and has an unstable resting potential. This unstable resting potential allows for spontaneous depolarization and gives the heart its autorhythmicity. It is the rate of change, or gradient, of the resting potential that determines the onset of the next action potential and therefore the discharge rate. The characteristics of the pacemaker potential are predominantly under the control of the autonomic nervous system.

An increase in the gradient of the slope of phase 4 will reduce the amount of time taken for the cell to reach threshold potential, causing depolarization to occur more rapidly. This occurs with sympathetic stimulation (red trace) via β1 adrenoreceptors which results in an increase in cyclic-AMP levels, allowing the opening of calcium channels and thereby increasing the discharge rate of the cell.

Conversely, a decrease in the slope of phase 4 will increase the time taken to reach threshold potential and depolarization, causing a reduced discharge rate. This occurs with parasympathetic stimulation (blue trace). The vagus nerve acts to slow the discharge rate by hyperpolarizing the cell membrane through increased permeability to potassium. The membrane potential is therefore more negative so will take longer to reach threshold potential and to discharge.

1.1.4

Cardiac cycle

The diagram depicts events that occur during one cardiac cycle. It is a graph of pressure against time and includes pressure waveforms for the left ventricle, aorta and central venous pressure (CVP), with the electrocardiogram (ECG) and heart sound timings superimposed.

There are five phases.

 Phase 1 (A). Atrial contraction – ‘P’ wave of the ECG and ‘a’ wave of the CVP trace. Atrial contraction (or ‘atrial kick’) contributes to about 30% of ventricular filling.

 Phase 2 (B). Ventricular isovolumetric contraction (IVolC) – marks the onset of systole and coincides with closure of the mitral and tricuspid valves (first heart sound). The pressure in the ventricle rises rapidly from its baseline, while blood volume remains constant, since both inlet and outlet valves are closed. The ‘c’ wave of the CVP trace represents tricuspid valve bulging as the right ventricle undergoes IVolC.

 Phase 3 (C). Systole – as the ventricular pressure exceeds that in the aorta and pulmonary arteries, the aortic and pulmonary valves open and blood is ejected. The aortic pressure curve follows that of the left ventricle, but at a slightly lower pressure, depicting the pressure gradient needed to allow forward flow of blood. At the end of this phase, ventricular repolarization is represented by the ‘t’ wave on the ECG.

 Phase 4 (D). Ventricular isovolumetric relaxation (IVolR) – once the aortic and pulmonary valves close (second heart sound), the ventricular pressure rapidly falls to baseline with no change in volume. Aortic valve closure is seen on the aortic pressure trace as the dicrotic notch, after which the pressure in the aorta exceeds that in the ventricle.

 Phase 5 (E and F). Ventricular filling – passive filling of the ventricle during diastole. As ventricular pressure falls below atrial pressure (and CVP), the tricuspid and mitral valves open allowing forward flow of blood. This filling is initially rapid (E), followed by a slower filling phase known as diastasis (F), before atrial contraction occurs and the cycle starts again. The ‘y’ descent on the CVP trace occurs as the atrium empties.

1.1.5

Cardiac output equation

Q = HR × SV

Q = cardiac output (ml.min–1)

HR = heart rate (beats.min–1)

SV = stroke volume (ml.beat–1)

Cardiac output (CO) is defined as volume of blood pumped by the heart per minute; it is equal to the product of heart rate and stroke volume. In considering this equation there are four determinants of CO: heart rate, preload, afterload and contractility. Changes in each variable do not occur in isolation but will impact the remaining variables. Therefore, depending on the magnitude of change, each variable may positively or negatively impact CO.

CO monitoring is frequently used as a means of optimizing tissue oxygenation and guiding treatment. Historically, the gold standard for CO measurement was invasive pulmonary artery catheterization. However, due to the specialist skill required for insertion and the potential for complications, its use has been superseded by less invasive methods.

 Pulse contour analysis (i.e. PiCCO, LiDCO) – algorithms relate the contour of the arterial pressure waveform to stroke volume and systemic vascular resistance. Research demonstrates good agreement with the gold standard. Limitations include the necessity for an optimal arterial pressure trace and potential for error (arrhythmias, aortic regurgitation).

 Oesophageal Doppler – estimates CO through measurement of blood velocity in the descending aorta (see Section 5.5 – Doppler effect).

 Transpulmonary thermodilution – based on the classical dilution method (dilution of known concentration of indicator injectate is measured within the arterial system over time) and is coupled with pulse contour analysis in the PiCCO system. Thermodilution is utilized to calibrate the PiCCO system and to provide measurements of volumetric parameters (i.e. global end-diastolic index) and extravascular lung water.

 Thoracic electrical bioimpedance (TEB) – a small electrical current is passed through electrodes applied to the neck and chest. The pulsatile flow of blood leads to fluctuations in current allowing calculation of CO from the impedance waveform. Studies have shown poor correlation between CO values derived via TEB and those dervived via thermodilution methods.

1.1.6

Central venous pressure waveform

The central venous pressure (CVP) waveform reflects the pressure at the junction of the vena cavae and the right atrium. It consists of three peaks and two descents:

 ‘a wave’ – the most prominent wave, represents right atrial contraction

 ‘c wave’ – interrupts ‘a wave’ decline, due to bulging of the tricuspid valve into the right atrium during right ventricular isovolumetric contraction (IVolC)

 ‘x descent’ – decline of right atrial pressure during ongoing right ventricular contraction

 ‘v wave’ – increase in right atrial pressure due to venous filling of the right atrium during late systole

 ‘y descent’ – decline of right atrial pressure as the tricuspid valve opens.

Alignment with the ECG trace may aid identification of the CVP waveform components.

 Onset of systole marked by ECG R wave; onset of diastole marked by end of ECG T wave.

 Three systolic components – ‘c wave’, ‘x descent’ and ‘v wave’.

 Two diastolic components – ‘y descent’ and ‘a wave’.

Potential errors in CVP measurement

Sampling errors: positioning of both the central venous catheter and the pressure transducer are important for accurate and precise measurement. Due to the narrow clinical range of CVP, small variations in the transducer reference point may have a disproportionally large effect on CVP measurement.

Interpretation errors: the effects of ventilation on CVP measurement must be considered. All vascular pressures should be measured at end-expiration, because pleural pressure is closest to atmospheric pressure. In positive pressure ventilation, low PEEP results in minimal error by only increasing the observed value by 1–2 mmHg. With high PEEP, error may be more difficult to predict.

1.1.7

Central venous pressure waveform – abnormalities

Examination of the CVP waveform may aid diagnosis of various pathophysiological conditions.

Cardiac arrhythmias

A – Atrial fibrillation is characterized by an absent ‘a wave’. The ‘c wave’ is more prominent due to a greater than normal right atrial volume at the end of diastole.

B – In isorhythmic AV dissociation, the atria and ventricles beat independently of each other but at the same rate. As such, the atria contract against a closed tricuspid valve producing an enlarged ‘a wave’ termed a ‘cannon a wave’.

Other arrhythmias also affect the CVP waveform. Sinus tachycardia is characterized by a shortening of diastole and therefore alters the diastolic waveform components (shortening of ‘y descent’ with merger of the ‘v’ and ‘a’ waves). In contrast, sinus bradycardia leads to increased distinction between the three waves.

Valvular disease

C – Tricuspid stenosis is a diastolic abnormality impeding right atrial emptying. As the right atrium contracts against a narrowed tricuspid valve, a prominent ‘a wave’ is produced. Right atrial pressure remains elevated for longer than normal, attenuating the ‘y descent’.

D – In tricuspid regurgitation, systolic flow of blood back into the right atrium through an incompetent valve leads to a persistent elevation of right atrial pressure. As such, the ‘c’ and ‘v’ waves gradually merge over time with subsequent loss of the ‘x descent’.

Elevation of CVP may be observed with raised intrathoracic pressure (positive-pressure ventilation), cardiac dysfunction (cardiac tamponade, cardiac failure) and circulatory overload.

Reduction in CVP may occur in association with reduced venous return (hypovolaemia, vasodilatation) and a reduction in intrathoracic pressure (spontaneous inspiration).

1.1.8

Einthoven triangle

Bipolar leads (I, II, III) electrically form an equilateral triangle named after Willem Einthoven, the scientist who developed the ECG. These leads, combined with unipolar augmented leads (aVL, aVR, aVF) examine the heart in the frontal plane. Rearranging these six limb leads, allowing an intersection representing the heart, forms the hexaxial reference system. The arrows represent the normal path of electrical current for each lead. This graphical representation of cardiac electrical activity aids interpretation of ventricular axis in the frontal plane.

Frontal ventricular axis determination

Normal cardiac electrical activity progresses systematically from the SA node, via internodal fibres to the AV node. Conduction continues via the bundle of His, through right and left bundle branches to Purkinje fibres, resulting in ventricular contraction. Depolarization towards a positive electrode produces a positive deflection on the ECG. When viewing the heart in the frontal plane, mean ventricular depolarization (as denoted by the QRS complex) lies between −30° and +90°. Ventricular axis may be determined using the limb leads. The simplest approach is the quadrant method, examining leads I and aVF. These perpendicular limb leads outline the majority of the normal axis.

 Normal axis – positive QRS complex in both leads.

 Extreme right axis deviation – negative QRS complex in both leads.

 Right axis deviation – negative complex in lead I, positive complex in aVF.

 Left axis deviation – positive complex in lead I, negative complex in aVF. However, as the normal axis ranges from −30° to +90°, this average vector may represent a normal axis. Examination of lead II is also required; if QRS complex is positive the axis is normal (ranging from 0° to −30°).

An alternative equiphasic approach exists, founded on the principle that depolarization travelling perpendicular to a lead produces an equiphasic QRS complex.

1.1.9

Ejection fraction equation

EF = SVEDV ×100

SV = EDV – ESV

EF = ejection fraction

EDV = end-diastolic volume

ESV = end-systolic volume

SV = stroke volume

The ejection fraction simply describes the amount of blood that is ejected from the ventricle during systolic contraction (stroke volume) as a proportion of the amount of blood that is present in the ventricle at the end of diastole (end-diastolic volume). A 70 kg individual would normally have a stroke volume of about 70 ml and an end-diastolic volume of about 120 ml.

The ejection fraction equation is used to calculate the stroke volume as a percentage of the end-diastolic volume. It gives an indication of the percentage of the ventricular volume that is ejected during each systolic contraction. It can be applied to the left or the right ventricles, with normal values being 50–65%. Right and left ventricular volumes are roughly equal and therefore ejection fractions are broadly similar.

In clinical practice, it can be calculated using echocardiography, pulmonary artery catheterization, nuclear cardiology or by contrast angiography.

In aortic stenosis, the ventricle will compensate for the increased obstruction to outflow by hypertrophy. This will initially maintain the ejection fraction and the pressure gradient across the valve. As the disease progresses and the valve area narrows, the hypertrophied ventricle becomes stiff and less compliant and will no longer be able to compensate. A reduction in the stroke volume (and ejection fraction) is seen, resulting in a fixed reduced cardiac output. The myocardium will eventually fail as compliance continues to worsen.

1.1.10

Electrocardiogram

An electrocardiogram (ECG) is a non-invasive, transthoracic interpretation of cardiac electrical activity over time. Thorough assessment requires a systematic approach including rate, rhythm, axis (normal axis is −30° to +90°), and wave morphology/interval.

Morphology and intervals

 P wave – represents atrial depolarization. A positive deflection should be present in all leads except aVR.

 PR interval – from the start of the P wave to the end of the PR segment. Normal value 0.12–0.2 s (3–5 small squares). This interval is rate dependent; as heart rate increases, the PR interval decreases.

 QRS wave – represents ventricular depolarization. The normal duration is ≤0.12 s. A Q wave in leads V1–V3 is abnormal.

 ST segment – from the junction of the QRS complex and the ST segment to the beginning of the T wave. A normal ST segment is isoelectric.

 T wave – represents repolarization of the ventricles.

 QT interval – from the start of the QRS complex to the end of the T wave. This interval represents the time for ventricular activation and recovery. Heart rate variability occurs and therefore a corrected QT interval (QTc) can be calculated (normal value is <0.44 s).

ECG changes associated with acute coronary syndromes and myocardial infarction

 Acute coronary syndromes – include non-ST-elevation myocardial infarction and unstable angina. The primary ECG changes observed are ST segment depression and T wave flattening or inversion.

 Myocardial infarction – early evidence of transmural ischaemia and myocardial infarction includes hyperacute T waves followed by ST elevation. Q wave formation may begin within 1 hour of infarction. Inverted T waves are a later sign within 72 hours of cell death. Stabilization of the ST segment usually occurs within 12 hours, although ST elevation may persist for more than 2 weeks.

1.1.11

Electrocardiogram – cardiac axis and QTc

Division of the hexaxial reference system into four quadrants allows further interpretation of the cardiac ventricular axis (for calculation see Section 1.1.8 – Einthoven triangle).

 The normal QRS axis ranges from −30° of left axis deviation (LAD) to +90°.

 LAD is defined as an axis between −30° and −90°. This may be an isolated finding or can be associated with pathology. Causes include: left ventricular hypertrophy, left bundle branch block (LBBB), left anterior fascicular block, myocardial infarction, and mechanical shifts of the heart (i.e. pneumothorax).

 Right axis deviation (RAD) is defined as an axis between +90° and +180°. Causes include: physiological variant in infants and children, right ventricular hypertrophy, myocardial infarction, left posterior fascicular block, chronic lung disease, dextrocardia, and ventricular arrhythmias.

 Extreme right axis deviation (ERAD) is defined as an axis of −90° to +180°. This is a rare finding associated with dextrocardia, ventricular arrhythmias or a paced rhythm.

Precordial axis

Assessment of the precordial leads, V1–V6, enables determination of the precordial axis as described by R wave progression. Normal R wave progression is characterized by a primarily negative QRS complex in V1 and a primarily positive QRS complex in V6. Transition between negative and positive complexes occurs between the V2 and V4 leads.

 Early R wave progression is characterized by much more positive QRS complexes in leads V1 and V2. This observation is always pathological and may be due to posterior myocardial infarction (with the positive QRS complexes representing reciprocal Q waves), right ventricular hypertrophy, RBBB, or Wolff–Parkinson–White syndrome.

 Poor R wave progression is characterized by a predominance of negative QRS complexes through the transitional precordial leads. This late transition can be a normal variant but may also be associated with anterior myocardial infarction, left ventricular hypertrophy, LBBB, or lung disease.

1.1.12

Fick method for cardiac output studies

Q = VO2Ca – Cv

Q = cardiac output (ml.min–1)

VO₂ = volume of oxygen consumed (ml.min–1)

C_a = oxygen content of arterial blood (ml O₂.ml blood–1)

C_v = oxygen content of venous blood (ml O₂.ml blood–1)

The Fick principle states that blood flow to an organ may be calculated using a marker substance if the amount of the marker taken up by the organ per unit time and the arteriovenous difference in marker concentration are known. This principle has been applied to the measurement of cardiac output (CO) where the organ is the entire body and the marker substance is oxygen.

 Direct Fick method – a minimum of 5 minutes of spirometry is required to determine resting oxygen consumption. During this time a peripheral arterial blood sample is obtained to calculate arterial oxygen content. Cardiac catheterization is required to calculate mixed venous oxygen content using a blood sample from the right ventricle/pulmonary trunk. A peripheral venous sample is insufficient because peripheral oxygen content varies markedly between tissues. This method is therefore time consuming and invasive. Validity is limited to the steady-state, prohibiting the use of this method during periods of changing CO such as exercise or other physiological stress.

 Indirect Fick method – application of the Fick principle through carbon dioxide rebreathing avoids invasive measurement of mixed venous oxygen content. Rebreathing techniques estimate arterial and venous carbon dioxide content through measurements of end-tidal partial pressure of carbon dioxide during normal breathing and intermittent rebreathing. Automated systems have eliminated much of the technical difficulty in performing this method.

 Thermodilution – based on the Fick principle, thermodilution is a minimally invasive method for CO measurement. The marker substance is a cold bolus of fluid and the arteriovenous difference is determined by a change in temperature. Thermodilution methods have been studied extensively and shown to correlate well with the direct Fick method. In addition to the minimally invasive nature of this method, other advantages over the direct method include validity during exercise and improved time resolution.

1.1.13

Frank–Starling curve

The Frank–Starling curve is used to represent the Frank–Starling law. It states that the ability of the cardiac muscle fibre to contract is dependent upon, and proportional to, its initial fibre length.

As the load experienced by the cardiac muscle fibres increases (within the heart this is the end-diastolic pressure, or preload) so the initial fibre length increases. This results in a proportional increase in the force of contraction due to the overlap between the muscle filaments being optimized. This intrinsic regulatory mechanism occurs up to a certain point. Past this, regulation is lost and contractility does not improve despite increasing fibre length, with eventual muscle fibre failure occurring.

A change in end-diastolic pressure (preload) will cause a patient to shift along the same curve. Increasing preload will cause the patient to shift up along the curve, resulting in increased cardiac output with each contraction. A reduction in preload will cause the opposite.

The whole curve can also be shifted as a result of inotropy or failure of the myocardium. An increased inotropy will cause a greater cardiac output for any given preload and therefore will shift the curve up and to the left. Failure of the myocardium will result in the curve shifting downwards and to the right, demonstrating that for any given preload the cardiac output will be reduced. There is a more exaggerated fall in cardiac output at higher preloads as the fibres become overstretched, with the curve falling off towards the baseline at the far right.

1.1.14

Oxygen flux

O2 flux (ml.min−1) = CO × [(1.34 × [Hb] × SpO2) + (PaO2 × 0.0225)]

CO = cardiac output

[Hb] = haemoglobin concentration (g.dl–1)

1.34 = maximal O₂ carrying capacity of 1 g of Hb measured in vivo (Hüfner’s constant) (ml.g–1)

SpO₂ = arterial haemoglobin oxygen saturation (%)

P_aO₂ = arterial oxygen tension (kPa)

0.0225 = ml of O₂ dissolved per 100 ml plasma per kPa

Oxygen flux is defined as the amount of oxygen delivered to the tissues per unit time. Oxygen delivery to the tissues is governed by two fundamental elements: cardiac output and arterial oxygen content. Arterial oxygen content comprises the sum of oxygen bound to haemoglobin and oxygen dissolved in plasma. The normal clinical range for oxygen flux is 850–1200 ml.min–1, with measurement requiring pulmonary artery (PA) catheter insertion.

Oxygen flux may be optimized, without invasive PA pressure measurement, if the modifiable variables are considered.

 Cardiac output (CO) – determined by heart rate, preload, contractility and afterload. These factors may be negatively affected by pathological states and drugs (i.e. anaesthetic agents, vasopressors). Optimization may include heart rate control, correction of volume status and administration of vasoactive medications. Direct treatment of disease states should also be implemented.

 Haemoglobin concentration – correction of anaemia will result in an increase in arterial oxygen content. Paradoxically, this may have a deleterious effect on oxygen flux due to the changing rheology of blood in the vascular compartment.

 Haemoglobin oxygen saturation (SpO2) – may be adversely affected by hypoxia due to hypoventilation, diffusion impairment and ventilation/perfusion inequality. Carbon monoxide poisoning and methaemoglobinaemia should be considered where appropriate. Optimization should focus on the use of supplemental oxygen to maximize alveolar oxygen tension (although the effect will be minimal in shunt) and specific treatment of the cause.

 Arterial oxygen tension – influences SpO2 and volume of oxygen dissolved in plasma. Increasing PaO2 has a finite effect on SpO2 once maximal saturation is reached. Dissolved arterial oxygen increases proportionally with an increase in PaO2. This increase becomes clinically significant at hyperbaric pressures.

1.1.15

Pacemaker nomenclature – antibradycardia

The pacemaker code has five positions.

 Position I – chamber paced.

 Position II – chamber sensed (detection of spontaneous cardiac depolarization).

 Position III – response to sensing on subsequent pacing stimuli.

 Position IV – presence or absence of an adaptive-rate mechanism in response to patient activity. The previous pacemaker code included a programmability hierarchy (i.e. simple vs. multi), which is now deemed unnecessary.

 Position V – presence and location of multisite pacing. This is defined as stimulation sites in both atria, both ventricles, more than one stimulation site in a single chamber or any combination of these.

Pacemakers and diathermy

If possible, diathermy should be avoided in patients with pacemakers. However, if diathermy is required, bipolar is safer (as the current travels between the two instrument electrodes). This should be used in short bursts at the lowest energy settings.

When diathermy is used intra-operatively, a variety of untoward events may occur. These include inappropriate pacemaker inhibition (failure to pace), system reprogramming, and permanent pacemaker damage. With the design of newer units, these events are becoming increasingly rare. The most frequent interaction is pacemaker inhibition caused by misinterpretation of diathermy electrical activity as intrinsic cardiac activity. If the pacemaker has a ‘D’ or ‘I’ in position III, the pacemaker becomes inhibited and does not pace. The clinical effect depends on the duration of electrical stimulus, the underlying cardiac rhythm and the degree of dependency on the pacemaker. Ideally, to avoid adverse diathermy interaction, pacemakers should be evaluated by a clinical electrophysiologist prior to surgery to develop a perioperative device management plan. This plan should consider re-programming to a fixed-rate mode, whether (and how) a magnet could be used and recommendation for follow-up post-operatively.

1.1.16

Pacemaker nomenclature – antitachycardia (implantable cardioverter-defibrillators)

The implantable cardioverter-defibrillator (ICD) code has four positions.

 Position I indicates the chamber shocked.

 Position II indicates the antitachycardia pacing chamber.

 Position III indicates the method of antitachycardia detection. Haemodynamic detection includes sensing of blood pressure or transthoracic impedance.

 Position IV indicates the antibradycardia pacing chamber, in case defibrillation results in bradycardia.

Perioperative considerations in patients with ICDs

 Preoperative – a multidisciplinary approach is essential. Perioperative management of an ICD should ideally be developed in collaboration with the cardiology and surgical teams, however, this may not always be feasible. In the out-of-hours setting, review of the patient’s information card and/or medical records should provide helpful information such as indication for treatment and functionality of the device. A CXR will help in determining the type of implantable cardiac device and if all leads are intact.

 Intraoperative – identification of potential sources of electromagnetic interference (EMI) is important to minimize device malfunction. Commonly encountered factors associated with EMI include electrocautery (diathermy), evoked potential monitors, nerve stimulators, and fasciculations. Generation of EMI may cause inappropriate defibrillation. To minimize this risk, antitachycardia functions should be suspended. Variability observed with magnet application to ICDs is less than that observed with pacemakers. For the majority of ICD devices, magnet application temporarily inhibits arrhythmia detection and discharge, with rapid resumption of antitachycardia functions with magnet removal. However, the use of a magnet, by non-experts, with certain devices may result in unanticipated results such as permanent programming changes, changes to antibradycardia functions or no change in function at all. Best practice advises perioperative input from an electrophysiologist or cardiologist. Continuous intraoperative haemodynamic monitoring and immediate availability of an external defibrillator are essential.

 Postoperative – continuous monitoring and external defibrillator availability must be continued until ICD function is resumed. A review by an electrophysiologist is recommended prior to termination of cardiac monitoring.

1.1.17

Preload, contractility and afterload

The definitions of preload, contractility and afterload were developed from experiments looking at isolated muscle fibres in vitro, allowing individual definitions to be produced. In vivo, these factors are interlinked, being dependent upon and affected by each other, making measuring them individually more difficult.

The stroke volume is determined by all three variables: preload, contractility and afterload. These, together with the heart rate, determine myocardial performance. Preload can also give an indication of how well a myocardium is performing. A heart requiring a higher preload to generate a cardiac output is not performing as well as one that is generating the same cardiac output with a lower preload.

In vivo, direct measurement of initial myocardial fibre length is not possible and therefore preload cannot be determined. As such, various surrogate markers have to be used. The volume in the ventricle at the end of diastole gives an indication of fibre stretch just before the onset of contraction. This can be measured by echocardiography and is called the end-diastolic volume.

The pressure generated in the heart chambers for a given volume is dependent on the chamber’s compliance, with the end-diastolic pressure often being referred to as the ‘filling pressure’. The right atrial pressure can be inferred from the CVP giving an indication of the filling pressure of the right side of the heart. The left-sided pressures are more difficult to measure and require a pulmonary artery catheter to obtain their values.

Contractility is difficult to define in isolation, being affected by all the factors that affect myocardial performance. Most factors that increase contractility do so by increasing the intracellular calcium concentration. Inotropy is often used synonymously with contractility.

Afterload can be represented by the mean arterial pressure during systole, or by measurement of the end-systolic pressure.

1.1.18

Pulmonary artery catheter trace

A pulmonary artery catheter is a balloon-tipped, flow-directed multi-lumen catheter, initially inserted through a central venous introducer sheath. During its placement a trace is produced demonstrating the pressures as the catheter moves through the chambers of the right heart and into the pulmonary circulation. The pulmonary capillary wedge pressure (PCWP) is used as a surrogate for the left atrial pressure.

Continuous pressure monitoring is used, via the distal lumen of the catheter, to guide correct insertion and produce the trace seen above. The balloon is inflated once the catheter has reached the right atrium and is allowed to float with the flow of blood to reach the pulmonary circulation. The right atrium pressure waveform is similar to the CVP waveform. On reaching the right ventricle the wave will oscillate between 0–5 mmHg and 20–25 mmHg. The catheter will then pass through the pulmonary valve and enter the pulmonary artery. The systolic pressure will remain the same as the right ventricle, but the diastolic pressure will rise to about 10–15 mmHg owing to the presence of the pulmonary valve. A PCWP is obtained by allowing the catheter’s balloon to occlude a pulmonary vessel. The trace will look similar to the CVP waveform, but with a range of 6–12 mmHg. The measurement should ideally be taken in West Zone 3 of the lung (where the pulmonary artery pressure is greater than both the alveolar and pulmonary venous pressures, ensuring a continuous column of blood to the left atrium) and at the end of expiration.

Pulmonary artery catheters can also be used to measure cardiac output (by means of an integral thermistor), mixed venous oxygen saturations, right-sided heart pressures and the right ventricular ejection fraction. It can also be used to derive systemic and pulmonary vascular resistances and the cardiac index.

1.1.19

Systemic and pulmonary pressures

The heart consists of two pumps in parallel: the low pressure right side that pumps into the pulmonary circulation, and the higher pressure left side that pumps into the systemic circulation.

The CVP approximates to the pressure in the right atrium and oscillates between 0 and 5 mmHg. In the right ventricle, the systolic pressure increases to 20–25 mmHg, with the diastolic pressure remaining similar to that in the right atrium. The presence of the pulmonary valve increases the diastolic pressure in the pulmonary artery to 10–15 mmHg, while the systolic pressure remains the same. The pulmonary capillary pressures are 6–12 mmHg, creating a pressure gradient that allows forward flow of blood from the pulmonary artery into the pulmonary circulation. The pulmonary capillary (wedge) pressure is often used as a surrogate for left atrial pressure and, in the presence of a normal mitral valve, left ventricular end-diastolic pressure. A pulmonary artery catheter allows accurate measurement of these pressures (see Section 1.1.18 – Pulmonary artery catheter trace).

The pressures in the left side of the heart are higher than the right due to the higher vascular resistance in the systemic circulation. To generate these higher pressures it therefore has a larger muscle bulk than the right. Left atrial pressure measures between 1 and 10 mmHg. During systole, the left ventricular pressure will rise to about 120 mmHg to generate forward flow. As blood passes through the aortic valve, the diastolic pressure will rise to about 60–80 mmHg with the systolic pressure remaining the same. Arterial cannulation can be performed to measure systemic pressures continuously. Peripheral cannulation will produce higher peak systolic and lower diastolic pressures than more central cannulation due to the differences in impedance and harmonic resonance. However, mean arterial pressure will remain broadly similar.

1.1.20

Valsalva manoeuvre

A Valsalva manoeuvre is performed by attempted expiration against a closed glottis. This results in an abrupt but transient increase in intrathoracic pressure and vagal tone. The normal physiological response to this manoeuvre consists of four phases.

 Phase I – sudden rise in intrathoracic pressure compresses capacitance thoracic vessels, increasing return of blood from the lungs to left atrium. A sudden, but transient, increase in systemic blood pressure is observed in accordance with the Frank–Starling law of the heart, coupled with direct compression of the thoracic aortic arch. Aortic arch baroreceptors are activated, initiating a compensatory reduction in heart rate.

 Phase II – venous return of systemic blood is impeded by sustained increase in intrathoracic pressure. This reduction in preload leads to a fall in cardiac output, once again, in accordance with the Frank–Starling law. A progressive reduction in blood pressure is observed. Baroreceptor activity is reduced, resulting in a sympathetically mediated gradual increase in heart rate, systemic vasoconstriction and a restoration of blood pressure.

 Phase III – sudden release of the intrathoracic pressure leads to an abrupt reduction of systemic blood pressure as compression of the aortic arch and thoracic capacitance vessels ceases. Baroreceptor activity is reduced, thereby maintaining heart rate elevation.

 Phase IV – an increase in blood pressure occurs with rapid restoration of the cardiac output as venous return suddenly increases. Systolic blood pressure exceeds the resting value (‘overshoot’) as blood is ejected into a constricted peripheral vascular system, as mediated by sympathetic activation in phase II. This rise in blood pressure results in baroreceptor activation and a compensatory bradycardia. Phase IV is not considered complete until the blood pressure has stabilized at its resting value. This may take up to 90 seconds and an ‘undershoot’ of blood pressure is often observed.

1.1.21

Valsalva manoeuvre – clinical applications and physiological abnormalities